In old worlds, world regrowth for Boulderbough did not work in the Fumarole biome.

In the prefab density settings, only tree_rock is present, while tree_rock1 and tree_rock2 are missing, so tree_rock1 and tree_rock2 will not undergo offspring regrowth in the Fumarole biome.

In TheWorld.generated.densities:

["FumaroleRetrofit:0:Chasm"] = {
  flower_cave_withered=0.041450777202073,
  flower_cave_triple_withered=0.018998272884283,
  flower_cave_double_withered=0.03454231433506,
  cave_vent_rock=0.11917098445596,
  retrofit_fumaroleteleporter=0.001727115716753,
  cave_fern_withered=0.089810017271157,
  shadowthrall_centipede_spawner=0.001727115716753,
  tree_rock=0.051813471502591
}

In map/static_layouts/retrofit_fumarole.lua, only tree_rock is included, so tree_rock1 and tree_rock2 are missing from the generated prefab density.

The relevant code location is as follows:

Quote

components\retrofitcavemap_anr.lua  -- line 943

map\object_layout.lua  -- line 184

map\static_layouts\retrofit_fumarole.lua  -- line 1